1. Low Band Receiving Loops
Design optimization and applications, including SO2R on the same band
Rick Karlquist N6RK

2. Topics
Small, square so-called “shielded” receiving loops for 160m and 80m.
Theory
Design and optimization
Applications
NOT: Transmit loops, delta loops, “skywire” loops, ferrite loopsticks, non-ham freq., mechanical construction

3. Why this presentation is necessary
Available literature on loop antennas is unsatisfactory for various reasons
Misleading/confusing
Incomplete
Not applicable to ham radio
Folklore
Just plain wrong (even Terman is wrong)
Even stuff published in Connecticut

4. The classic loop antenna

5. Any symmetrical shape OK

6. Loop antenna characteristics
Same free space pattern as a short dipole
Directivity factor 1.5 = 1.76 dB
Sharp nulls (40 to 80 dB) broadside
Much less affected by ground and nearby objects than dipole or vertical
Low efficiency (~0.1 to 1%), about the same as a modest mobile whip
Portable (no ground radials needed)

7. Why to use a receiving loop
Can null interference (QRM or QRNN)
Direction finding to locate QRNN
Remote receiving antennas
SO2R on the same band (160 meter contests, field day, SOSB, DXpeditions
Although vertically polarized, may be quieter than a vertical

8. Design equations: size, inductance
Maximum size side = wavelength
10 ft at 2 MHz; 5 ft at 4 MHz
ARRL Antenna Book inductance is wrong
L=0.047 s log (1.18s/d)
L=mH; s = side(in); d = conductor dia(in)
Reactance of max size loop = 226W for s/d = 1000, independent of frequency
Only weakly dependent on s/d

9. Conductor loss resistance
We will assume copper conductor
Conductor loss depends only on s/d
Conductor loss at 2 MHz = s/d
If s/d=1000, conductor resistance = .47W
Conductor loss at 4 MHz max size loop= s/d
If s/d=1000, conductor resistance = .66W

10. Radiation resistance Radiation resistance = (FMHZs/888)4
For max size loop, Rr = ohms, independent of frequency
At 2 MHz, Rr = (s/444)4
At 4 MHz, Rr = (s/222)4
Radiation resistance is negligible compared to conductor loss

11. Loaded Q; efficiency
For maximum size loop, s/d = 1000, theoretical QL = 2 MHz, 4 MHz
Theoretical efficiency h = 1.4% ( MHz; 0.97% (-20.1 dB) at 4 MHz
Gain will be higher by 1.76 dB directivity factor
Doubling s increases efficiency 9 dB
Doubling d increases efficiency 3 dB

12. Maximum circumference
No definitive explanation of where this number comes from is published AFAIK
In a “small” loop, current is uniform everywhere in loop
As loop size increases, current phase becomes non uniform
For large loops current magnitude is also non uniform

13. Effects of “large” loop
Supposedly, a too-large loop will have poor nulls, but is this really true?
For vertically polarized waves, there is a broadside null for any size, even a 1 wavelength “quad” driven element
For horizontally polarized waves, there is an end fire null for any size
Topic for further study
I will use ARRL limit of wavelengths

14. Multiturn loops
Maximum perimeter rule applies to total length of wire, not circumference of bundle
To the extent that max perimeter rule applies, multiturn configuration greatly limits loop size
Multiple turns are a circuit design convenience, they do not increase loop sensitivity
Multiple turns in parallel make more sense
We will assume single turn from now on

15. Imbalance due to stray C

16. The classic “shielded” loop

17. So-called “shielded loop”
First described (incorrectly) in 1924 as “electrostatic shield” and repeated by Terman
If the loop were really an electrostatic shield, we could enclose the entire loop in a shield box and it would still work; we know that is false
Theory of shielded loop as published overlooks skin effect
Shielded loop actually works and is useful, but not for the reasons given in handbooks

18. Disproof of electrostatic shield

19. Development of classic loop into “shielded” loop

20. 1. Make conductor a hollow tube

21. 2. Add feedline to RX

22. 3. Change line to tandem coax

23. 4. Re-route coax through tube

24. 5. Swap polarity of coax

25. 6. Delete redundant tubing

26. 7. Add feedline to RX

27. 8. Feedline isolation transformer

28. 9. Relocate tuning capacitor

29. Coax capacitance
Capacitance of coax is in parallel with tuning capacitor
The two coax branches are effectively in series so the capacitance is halved
Use foam dielectric 75 ohm coax to minimize loss of tuning range
Still possible to reach maximum frequency where perimeter = wavelengths

30. Complete design, fixed tuning

31. Example 160/80m loop

32. Example, max size 160/80 loop Total length of coax, 20 ft
Perimeter is wavelength at 4 MHz
Bandwidth ~25 to 50 kHz
Gain 20 to 30 dB below transmit vertical
Tuning capacitance pF
Loop impedance ~ 5000 ohms
Transformer turns ratio ~50:5

33. Matching transformer
Use a transformer, not a balun, this is not for transmit.
Use low permeability core (m=125), Fair-Rite 61 material, 3/8” to ½” diameter
Use enough turns to get 100 mH on the loop side, typically 50T on 3/8” high core
Wind feedline side to match to 50 or 75 ohm feedline, approx. 5 turns
This core has negligible signal loss
Do NOT use high perm matl (73, 33, etc)

34. Remote varactor tuning
Use AM BCB tuning diodes
Only source of new diodes to hams is NTE618 (available Mouser and others)
Continuous tuning from below 1.8 MHz to above 4 MHz
Tuning voltage 0 to +10V

35. Remote tuning circuit

36. Strong signal issues Typically no BCB overload problem
No problem 6 miles from 50 kW station
Make sure birdies are in antenna, not your receiver
In case of a problem, use strong signal varactor circuit
For SO2, may need to avoid varactors altogether

37. Strong signal circuit

38. Loop size issues Bandwidth (counterintuitively) is independent of size
Tuning cap inversely proportional to loop width
Gain increases 9 dB (theoretically) for doubling of loop width
I observed more than +9 dB for full size loop on 160 meters (14 ft wide) vs 7 foot wide
Doubling conductor diameter increases gain 3 dB, halves bandwidth
Nulling still good on large loops

39. Sensitivity issues Noise from antenna must dominate receiver noise.
Example loop was quite adequate for FT1000; even a half size loop was OK.
For 160 meter remote loop at long distance, consider 14 foot size. Easier than a preamp

40. Applications Nulling power line noise, good for several S units
Very useful for DF’ing power line noise
Get bearing then walk to source using VHF gear to get actual pole
Remote loop away from noise if you have the land
Compare locations for noise using WWV(H) on 2.5 MHz as a beacon
Null your own transmitter for SO2R

41. 2007 Stew Perry SO2R setup

42. SO2R results Transmitted on 1801 kHz (the whole contest!)
Receive (while transmitting) > 1805 kHz
Transmit rig FT1000, SO2R rig TS-570
Nulling is weird near shack, inv V, or OWL
Location used was near 60x40x16 metal building
60 to 80 dB nulling. Angle tolerance a few degrees
Able to hear about everything. CE/K7CA was a few dB worse than beverage

43. CU on the low bands
73, Rick N6RK